Vesicle transport is the general process in eukaryotic cells by which proteins synthesized in the endoplasmic reticulum (ER) are transported via the Golgi network to the various compartments in the cell where they will function. Other proteins are transported to the cell surface by this process where they may be secreted (exocytosis). Such proteins include membrane bound receptors or other membrane proteins, neurotransmitters, hormones, and digestive enzymes. The transport process uses a series of transport vesicles that shuttle a protein from one membrane-bound compartment (donor compartment) to another (acceptor compartment) until the protein reaches its proper destination (Rothman, J. E. and Wieland, F. T. (1996) Science 272:227-34).
The process begins with the budding of a vesicle out of the donor membrane. The vesicle contains the protein to be transported and is surrounded by a protective coat made up of protein subunits recruited from the cytosol. The initial budding process and coating processes are controlled by cytosolic GTP-binding proteins (GTPB). When GTP binds and activates the GTPB, the GTP-GTPB complex binds to the donor membrane and initiates the vesicle assembly process. The coated vesicle containing the GTP-GTPB complex detaches from the donor compartment and is transported through the cytosol. During the transport process, the GTP is hydrolyzed to GDP, and the inactivated GTPB dissociates from the transport vesicle and is recycled. At this point, the protective coat of the vesicle becomes unstable and dissociates from the enclosed vesicle. The uncoated vesicle is recognized by its acceptor compartment through exposed surface identifiers (v-SNAREs) which bind with corresponding molecules on the acceptor compartment membrane (t-SNAREs). The transport process ends when the vesicle fuses with the target membrane.
Many of the proteins involved in synaptic vesicle transport have been identified and the biochemical interactions between them have been characterized. Interestingly, many of these proteins are homologous to yeast proteins involved in yeast secretory pathways. For example, BET1, BOS1, and SEC22 form a set of interacting yeast genes required in endoplasmic reticulum (ER) to Golgi protein transport (Newman, A. P. et al. (1992) EMBO Journal 11: 3609-17). Each of these genes encodes a small integral membrane protein with hydrophilic, cytoplasmic N-terminal and central regions, and a C-terminal hydrophobic transmembrane anchor. Mammalian counterparts of yeast bet1 and sec22 have been identified (Hay, J. C. et al. (1996) J. Biol. Chem. 271: 5671-79). Both proteins are widely expressed in mammalian tissues, as would be expected of proteins involved in fundamental membrane trafficking reactions. Mammalian bet1 and sec22 are localized to the ER membrane and the Golgi membrane, respectively, indicating that they may participate in vesicle transport in opposite directions between the ER and the Golgi apparatus (Hay et al. supra).
GTP-binding/GTPase proteins are likewise essential in both yeast and mammalian secretory pathways. At least four genes encoding GTPB's have been shown to be involved in transport between ER and the plasma membrane. These include SEC4, YPT1, SAR1, and ARF1 (Dascher, C. et al. (1991) Mol. Cell. Biol. 11:872-85). Yeast sec4 GTPB, essential for late stages of vesicle secretion, is homologous to mammalian Ra3a GTPase. The Rho proteins are another small family of GTPBs found in both yeast and mammals (Nakano, K. and Issei, M. (1995) Gene 155: 119-22). Rho proteins regulate the actin cytoskeleton during cell division and control signal transduction by linking receptors of growth factors to actin polymerization. RhoD is a member of the mammalian Rho family that provides a link between membrane trafficking and cytoskeleton regulation. RhoD causes rearrangements of the actin cytoskeleton and cell surface, and governs endosome motility and distribution (Murphy, C. et al. (1996) Nature 384: 427-32).
GTP-binding proteins share several amino acid sequence motifs, termed motifs I-IV. Motif I has the signature, GXXXXGK. The lysine residue is essential in interacting with the .beta.- and .gamma.-phosphates of GTP. Motif II, III, and IV are highly conserved, with DTAGQ, NKXD, EXSAK/L as their respective signatures. These motifs regulate the binding of .gamma.-phosphate, GTP, and the guanine base of GTP, respectively. In addition, Rho proteins have a Cys-aliphatic residue-aliphatic residue-X (CAAX) box for the binding of a prenyl group and either a palmitoylation site or a basic amino acid-rich region, suggesting their role in membrane-associated functions.
The discovery of new vesicle transport associated proteins and the polynucleotides encoding it satisfies a need in the art by providing new compositions which are useful in the diagnosis, prevention and treatment of cancer, and immune, reproductive, and developmental disorders.